Previous case reports and studies have shown that anti-myelin-associated glycoprotein (MAG) antibody can be detected in patients with polyneuropathy without any detectable M-protein. Nevertheless, the frequency of and related factors have not yet been adequately investigated. The objectives of this study are to examine the prevalence of anti-MAG antibody in patients with demyelinating neuropathy without M-protein and to determine their clinical characteristics. From January, 2004, to September, 2016, consecutive patients with chronic demyelinating neuropathy were recruited. Anti-MAG antibody presence was tested at the first evaluation. We determined the prevalence of anti-MAG antibody without M-protein among included patients and evaluated the clinical characteristics. A total of 44 patients were included in the present study (12 women; median age at first visit 60 years [interquartile range 47-67 years]; median duration between onset and first visit 9 months [3-26 months]). M-protein was found in eight patients (18%) at the first evaluation. Anti-MAG antibody was present in 2 of remaining 36 (5.6 [95% confidence interval 0-13.0] %) patients without M-protein. Patients with anti-MAG antibody exhibited slowly progressive and distal dominant neuropathy with elevated serum IgM levels and refractory to immunotherapy. There were no differences in clinical features between patients having anti-MAG antibody without M-protein, and those with M-protein. One patient with the anti-MAG antibody showed a delayed appearance of M-protein during a 4-year follow-up after diagnosis. The prevalence of the anti-MAG antibody in chronic demyelinating neuropathy without any detectable M-protein was 5.6%. Anti-MAG antibody may be detectable earlier than M-protein.